The present invention relates to a packaging in which food can be transported and/or sold which is intended for consumption in a heated state and is to be heated in the packaging.
The invention also relates to a method for cooking food which is at least partly fresh in a container in a microwave oven whereby during the heating process a vapor banking situation with excess pressure occurs within the container due to the water content of the food as well as the design of the container.
Amongst the known packagings of the previously mentioned type, in particular the boilable bag deserves to be mentioned with regard to dry food which can be preserved freshly in this condition for a relatively long time period and which is usually cooked in water prior to consumption, for example rice. In order to permit the hot water for blanching to reach the food, boiling bags mostly feature a multitude of small holes or are designed permeably in some other way.
Particularly in the field of convenience food a multitude of bags, dishes and other containers for moist food exist which are mostly enclosed in order to protect the food which is more perishable due to its inherent moisture from external influences, to keep the food fresh, to protect it from dehydration or simply to prevent any liquid spillage. Regularly in this connection the foodstuffs are ready-cooked so that they only need to be heated in a bain-marie, a baking oven or a microwave oven, for instance. This is called xe2x80x9cregeneratingxe2x80x9d in contrast to cooking food from its fresh state. A prior opening of the packaging by cutting it open or removing a cover for instance, is mostly required in this-connection and therefore also mandatory.
In relation to the above mentioned method it is known that fresh food with an inherent water content such as vegetables has to be cooked in a microwave oven using cooking utensils which are especially designed for this purpose. The known cooking utensils are however unsuitable as packaging in which food can be sold and/or transported and would pricewise be too costly to be used for this purpose.
Moist food cannot be cooked in open containers using a microwave oven, or it can at least not be cooked in a microwave without further precautions. Particularly vegetables tend to dehydrate in an open container. Even the tearing of meat fibers can occur when using meat.
According to the invention as characterized in claim 1 a packaging is provided in which food of the aforementioned type can be transported and/or sold, said packaging is characterized in that it consists of a microwaveable material, which by taking into consideration the water content of the food, an amount of liquid is contained in the packaging such that the food can be heated in the vapor atmosphere developing inside the packaging using a microwave oven, and that the packaging is provided with means for limiting and/or reducing the vapor pressure which builds up inside the packaging during rapid heating in the microwave oven.
Using the aforementioned means for limiting and/or reducing the vapor pressure, more preferably it is also possible to use the packaging for fresh foods and actually cook these in a microwave oven although for this, depending on the specific consistency of the food, a not inconsiderable amount of energy or power is possibly required. The means denoted however ensure even in this case that the packaging is not destroyed by the vapor pressure which builds up even during rapid heating in a microwave oven, and by the resulting possibly fairly high vapor temperature. The packaging according to the invention is therefore especially designed and suited for the preparation in minutes of food contained in the packaging from the fresh state to its cooked condition, being ready for consumption, using a microwave oven.
Due to the vapor pressure which builds up and is higher than the ambient pressure, and the temperature thereby increased beyond 100xc2x0 C. the cooking process is effected rapidly and gently. For example, a mixture of fresh vegetable pieces can be cooked ready for consumption within approx. 3 minutes in a 750 W microwave oven. Surprisingly in this connection even a type of plateau phase with respect to the cooking condition eventuates with regard to several commonly used vegetables such as zucchini which means that the said vegetables remain in a practically optimal cooking condition for a certain time period and that other vegetables which cook slower, such as potatoes, have time to reach the desired cooking level, an effect which for example does not occur when the same vegetable varieties are cooked in water or are vaporized in a pan. Using the traditional methods of preparing these foods the various vegetables are mostly cooked in graduated time intervals in order to compensate for the different cooking times required, and are additionally mostly cut into different sizes. As an example, for a typical traditional ratatouille, carrots are cut 0.5 mm thick, cauliflower and broccoli 0.7-0.8 mm, onions and leeks 10 mm and pepper, zucchini or eggplants 15-20 mm. For the preparation in the packaging according to the invention the very same vegetable varieties can practically be cut to equal thickness, for example 10 mm, with a variation in cut depth of only 20% due to the aforementioned plateau effect. An equal size of the various vegetables is optically more appealing than the method used traditionally.
A piece of fresh chicken breast weighing 200 g can also be cooked within only 3xc2xd minutes using the packaging according to the invention. A piece of fresh fish fillet of equal size requires just one minute.
In comparison with traditional cooking methods the foods, particularly vegetables, prepared according to the present invention also retain their natural aroma to a greater extent.
The advantages achieved by using the invention can furthermore be considered in that
no additional cooking utensils are required and therefore the amount of dish washing is reduced;
special cooking utensils suitable for rapid cooking in a microwave oven, like the aforementioned, are not required to be purchased and stored readily available in the kitchen thereby taking up storage space;
the packaging, particularly if this represents a dish, can also be used as a plate substitute;
no special knowledge is required for the preparation;
the food contained in the packaging can be cooked ready for consumption in a matter of minutes;
the preparation can practically be carried out without adding any fat or additional liquids provided that the food has a sufficiently high inherent water content of at least 30%;
because of the possibility of preparing food in its fresh condition the said food shows higher quality in terms of consistency and nutrient content in comparison to commonly used ready-cooked convenience food;
the cooking or regenerating can also be done in a baking oven or bain-marie if for example a microwave oven is not available, whereby naturally the possibility exists to take the food out of its packaging prior to preparation and to prepare it the conventional way in commonly used cooking and frying utensils.
The means for limiting and/or reducing the vapor pressure are preferably designed in such a way that they enable the vapor which is building up in the packaging during the rapid heating process per unit of time in a microwave oven to escape comparatively slower from the packaging into the ambient atmosphere mainly due to the flow resistance of the said means. These means can for example employ a valve which opens preferably automatically to the outside, located in a wall of the packaging, or just a comparatively small ready-made and therefore well defined hole. In the latter case it is preferable to cover the hole for the sale and/or transport of the foodstuffs with an adhesive label which can be removed by the consumer prior to heating in a microwave oven. The adhesive label has to be sufficiently adhesive so that it cannot become detached due to any fluctuations in pressure occurring within the packaging during transport and storage. A valve that opens automatically offers the advantage of greater user safety compared to a hole covered by an adhesive label. For example, a valve as used under the name of WICOVALVE for filling bags for roasted coffee made by the company Wipf AG in CH-8604 Volketswil, and which opens to the outside at an only marginally excess pressure less than 5 mb, whereas it provides a good seal in the opposite direction, is suitable. After the valve has opened due to internal excess pressure it closes again automatically when the excess pressure has fallen to a value of 0.5 mb. These valve properties also ensure that the excess pressure which builds up during the heating phase in the packaging can again be reduced rapidly and completely after the heating process, and a possible deformation of the packaging can at least largely return to its original shape largely so that the packaging while still hot can be opened without any risk immediately after its removal from the microwave oven.
In terms of the selection and dimensioning of the aforementioned means for limiting and/or reducing the vapor pressure as well as their flow resistance the said means naturally have to be adapted to the size and stability of the packaging, to type, properties, water content and reaction to cooking of the contents in the packaging as well as to the calorific output of the microwave oven. Another influential factor is the size of the packaging in relation to the volume of the contained food. Preferably all these parameters are matched and optimized for a specific product whereby, furthermore, preferably the same packaging and pressure control means are used for different food types, if possible. The preferable heating capacity to be selected is such that it can be set directly using standard microwave ovens. Microwave appliances used in a household situation offer for example 600 W or 750 W. Microwave appliances used in a commercial situation can be operated to up to 1500 W. The easiest way is to vary the cooking time from meal to meal.
In order to conclusively predetermine a cooking time for a particular recipe to achieve a certain desired result (for vegetables for example al dente or à point) the remaining parameters need to be kept within narrow boundaries. This applies in particular to the means utilized for limiting and/or reducing the vapor pressure. The previously mentioned valve made by the company Wipf has again proven itself to be suitable because its opening pressure as well as its flow resistance are very well defined. The valve has a dimensionally stable valve gate in which a small membrane is securely situated. Inside the bottom part of the valve body, to be more precise inside the valve face area provided there, several well defined holes are incorporated which are covered by the said membrane. The holes are surrounded by ring-shaped grooves which serve as the intake for a sealing oil. The membrane is secured in a center part of the valve face area using an armature component exerting a certain pre-stress. For a sufficiently accurate specification of the cooking time, for example, so-called foil valves might be too inaccurate, said foil valves consisting of two foil walls of the packaging which are jig welded together and possibly additionally equipped with rippled embossing in such a way that a ventilating channel equipped with baffles results. The internal excess pressure necessary for this channel to open is not easily controllable and may also not be much below 20 mb particularly when it is considered that the foils cannot be too thin due to stability problems. In addition, a sufficiently safe and hygienic seal is at least difficult to realize using such valve designs. For the aforementioned valve made by the company Wipf as well as for other known valves used for food packagings, the presence of a sealing oil plays a not inconsiderable role in order to provide a safe seal. Finally, foil valves of the previously mentioned type cannot simply be placed where they, on the one hand, do not interfere with the filling and sealing of the packaging, but on the other hand are located in the most optimal way for the cooking process.
It becomes apparent how susceptible the selection of the correct flow resistance of the pressure control means is, if for example two valves instead of just one of the described type are used. Because of this the cooking time increases not inconsiderably due to the lower excess pressure building up inside and the lower temperature thereby achieved so that for example zucchini do not reach their optimal cooking point within a time period set for one valve. If, on the other hand, the cooking time was predetermined correctly for two valves but packaging with one valve were to be used, the zucchini would already be overcooked on reaching the end of the longer time span.
Vice versa it is also obvious from this, that the selection of the optimal cooking time for a particular recipe can itself be a critical factor. However, by having designed the packaging according to the invention as packaging in which food can be transported and sold it is possible on the part of the manufacturer to determine the optimal cooking time for a particular recipe while taking into consideration all remaining factors such as the heating capacity, physical properties of the selected packaging etc in preliminary tests and indicate this on the packaging so that the end-user is not left searching for the correct settings.
The magnitude of the flow resistance of the pressure control means is eventually also affecting the moisture content of the pre-prepared vegetables. If the cross section of flow is too large more vapor is allowed to escape and at the same time a longer cooking time is necessary due to the lower temperature and lower pressure. Because of this undesirable effect, at least superficial dehydration of the food can eventuate.
The packaging is preferably designed as disposable packaging for once only use and should therefore be economical in terms of manufacturing and material usage. This can be especially achieved by utilizing a commonly used thin-walled synthetic material, for example in the shape of molded plastic dishes sealed by a cover foil, or just by bags made from commonly used synthetic foil or similar materials.
Naturally the packaging material and the packaging as a whole have to withstand the excess pressure developing during the vapor banking phase as well as the temperatures thus reached. Towards the end of the cooking time temperatures of up to 150xc2x0 C. an excess pressure of up to 3 bar can also be reached. The suitability of the material to be used in microwave ovens has been mentioned earlier.
Under no circumstances is the material permitted to melt under the influence of high temperatures or change in any other negative way for the food. It also has to be sufficiently dimensionally stable so that the packaging cannot expand to the ceiling or walls of the cooking displacement of a microwave oven which is used in accordance with the invention, and cannot virtually fill out this cooking displacement. In this connection the means for limiting and/or reducing the vapor pressure could for example come in contact with a wall or the ceiling of the cooking displacement and therefore cause a hindrance for the vapor emission and possibly a resulting bursting of the packaging. For example, microwave appliances designed for a household situation only the material offer a usable height of less than 20 cm of the cooking displacement.
The material should furthermore be as elastically reversible as possible so that the packaging does not deform in a permanent way during the heating and excess pressure phase, and possible deformations can return to their original size again as completely as possible. This demand is above all important for saucer-type containers which may possibly be used as plate substitutes whereby in this case the bottom part is of utmost importance.
At least the bottom of the saucer-type container should return as evenly as possible after the heating and pressure load so that the container stands evenly on a surface and does not wobble. A suitable shaping of the saucer-type container can additionally counteract an excessive deformation and/or support a return to the original shape. The use of fluting of the side walls achieves for example a type of advantageous concertina effect.
In terms of temperature stability polypropylene or CPET are suitable materials, for example. The melting point of polypropylene occurs at around 160xc2x0 C., and this material can be heated to 110xc2x0 C. continuously. Short-term even temperatures of up to 140xc2x0 C. are possible. The melting point of CPET occurs at approx. 264xc2x0 C. but it is substantially less elastic than polypropylene and also does not return as well as polypropylene back to its original shape after the heating and pressure load. Customary polyethylene otherwise mainly used for food packaging is hardly suitable because even the high density polyethylene HD-PE reaches its melting point at 135xc2x0 C., and therefore the short term and especially the long term stability under load are not located in the range required for the present invention.
In order to be able to withstand the heat and pressure the material that is chosen should preferably not be too thin. For example, for a dish made of polypropylene wall thicknesses in the range between 500 xcexcm and 1000 xcexcm are suitable. Below 500 xcexcm it becomes critical with respect to stability. Above 1000 xcexcm the deformations which occur during the vapor banking phase no longer return to size sufficiently. For bags such as bags with a solid bottom part, the wall thicknesses should still be 100 xcexcm to 200 xcexcm whereby these can possibly be additionally reinforced if polypropylene is being used by laminating them together with a possibly pre-stretched foil with limited extensibility made from different materials such as nylon.
In order to enable an impervious packaging seal after the filling of the packaging in a simple, rational and commonly used way at least a portion of the material should be sealable, i.e. it should be able to be jig welded by applying pressure and heat. Provided that this requirement appears to be in contrast to the remaining requirements, especially in regard to compression, resistance laminates consisting of several layers which offer different properties can be used, for instance. A laminate that is suitable as a cover foil for the sealing of dishes made of polypropylene is one example which is manufactured from a 75 xcexcm polypropylene foil and a 12 xcexcm polyester foil whereby the polypropylene foil ensures the weldability and the polyester foil laminated to it provides sufficient stability. In order to be of sufficient stability a pure polypropylene foil would have to be made of such a thickness that it would not be sealable, or at least not by using customary heat sealing methods and devices. Instead of laminating additional foils to it, sealable foils could also be stabilized in different ways, for example by joining them to network structures or similar.
The usage of a material, at least in part, which is as transparent as possible is preferred for the packaging, particularly for example for the cover foil of dishes, so that the customer purchasing the packaging can recognize the contents easily.
Finally, the materials used for the packaging should also be as gastight as possible.
For the desired vapor atmosphere to develop inside the packaging during the heating process the food should have a water content of at least 30%, preferably even 40%.
The food is preferably pre-seasoned ready for consumption so that it can be consumed immediately after the heating process in the microwave oven. The xe2x80x9cseasoningxe2x80x9d includes the commonly used flavors but also the addition of, for example, a small amount of herb butter, a sauce or similar. Meat or fish pieces are preferably covered with emulsified seasoning, a marinade containing oil and fat or similar which also protects the said pieces from becoming stringy when heated rapidly in a microwave oven.